Question

A person can plow out all the driveways on his street with his new four-wheel-drive truck in 4 hours. using a snow blade on a lawn tractor, his neighbor can plow out the same number of driveways in 10 hours. how long would it take them to do the work together?

Answer 1

c = combined time

they each do a fraction of the work
... the fractions add to one...the whole job

(c / 4) + (c / 10) = 1

20 is the common denominator ... (5c / 20) + (2c / 20) = 1

5c + 2c = 20 ... c = 20 / 7